The present invention relates to hydrated calcium aluminate expansive materials and more particularly, the present invention relates to shrinkage compensating admixtures for mortar, concrete and cementitious construction products.
Contraction or shrinkage is an inherent characteristic of cementitious materials with concomitant cracking and other related structural failures. Three broad categories characterize shrinkage, namely: plastic, autogenous and drying. In order to minimize shrinkage strains, shrinkage reducing admixtures (SRA) and shrinkage compensating admixtures (SCA) are incorporated in the cementitious materials. Although SRA reduces shrinkage strains, its effectiveness is compromised in freezing and thawing conditions and its use is limited because it is a liquid component. Calcium sulfoaluminate based admixtures are the most effective SCA. These additives promote expansion in the mix that counteracts shrinkage deformations. They are largely used in cementitious mixes, allowing for good flexibility in the formulation of cementitious materials, at a lower cost, and can be dry blended with cement. These materials have utility in certain circumstances, but are limited in freezing and thawing conditions. One of the more commonly used SRA compositions is calcium sulfoaluminate admixture (CSA). This additive is useful for enhancing flexibility in the formulation of cementitious materials at a lower cost and also facilitating the possibility of dry blending the mixture with cement.
In the prior art, Sheikh, U.S. Pat. No. 5,741,357, issued Apr. 21, 1998, provides a hydrated high alumina cement. This reference teaches that the inner core of the particles is the anhydrous alumina bearing material and is accordingly a source of aluminates necessary the formation of ettringite and subsequent expansion. The outer coat of hydrated products prevents the rapid dissolution of the inner core material to control the formation of ettringite and the timing of expansion. Although this technology is quite meritorious, it was somewhat limited in effectiveness of the prehydration process in preventing the accelerated reaction of the high aluminous cement and the inconsistency produced in relation to the degree and timing of expansion.
One of the primary limitations of the technology in U.S. ""357 relates to the fact that the particles are ground for the production of the admixture. As will be appreciated, due to the grinding, a mixture of particles having differing amounts of unhydrated product unprotected by the hydrated crust naturally result (FIG. 1). This subsequently translates into potential inconsistency in the rate and amount of expansion obtained by the admixture. Such a limitation can compromise the performance of the admixture as an effective shrinkage compensating agent.
Having regard to the progress made in this field as outlined in the Sheikh patent, the present application overcomes the admixture homogeneity problem and teaches an admixture that can be dry blended with cementitious material to provide a homogenous final product.
An objective of all embodiments of the present invention is to provide an improved cementitious additive and mixture incorporating the additive for use in cementitious compounds to alleviate the complications and limitations in the compositions of the known art.
It is an object of one embodiment of the present invention to provide an expansive additive for controlling shrinkage in a cementitious mixture, comprising:
calcium aluminate particles where at least 75% by mass of the particles are hydrates of C3AH6 CAH10 or a mixture thereof, where C represents CaO, A represents Al2O3, and H represents H2O;
an expansion effecting sulfate-based compound for effecting expansion in the mixture; and
calcium hydroxide.
A further object of one embodiment of the present invention is to provide a cementitious mixture having reduced shrinkage upon curing, comprising:
cementitious material;
calcium aluminate particles where at least 75% by mass of the particles are hydrates of C3AH6, CAH10 or a mixture thereof, where C represents CaO, A represents Al2O3, and H represents H2O;
an expansion effecting sulfate-based compound for effecting expansion in the mixture; and
calcium hydroxide.
In the prior art, there is no teaching with respect to the use of a high by mass concentration of C3AH6 hydrates, CAH10 hydrates or a mixture of these as an addition for a cementitious mixture. In fact, the prior art methods and compounds are focused upon anhydrous particles. Fu, et al., in xe2x80x9cCharacteristics of Shrinkage Compensating Expansive Cement Containing a Pre-Hydrated High Alumina Cement-Based Expansive Additivexe2x80x9d, Cement and Concrete Research, Vol. 24, No. 2, pp 267-276, 1994, are concerned with the use of the anhydrous material to effect desirable results.
In the present invention, it has been found that the additive can effectively be customized for specific uses/environments of use and, therefore, is not limited to a specific field of utility, but rather is useful in a host of cementitious mixtures.
A still further object of one embodiment of the present invention is to provide a method for synthesizing hexagonal phase calcium aluminate hydrate particles, comprising the steps of:
i) providing a source of calcium aluminate particles;
ii) hydrating the calcium aluminate sufficiently to form a slurry;
iii) agitating the slurry sufficiently for hydration to occur and form hexagonal phase calcium aluminate hydrate particles;
iv) maintaining the temperature of the slurry to prevent thermal conversion of the hexagonal phase calcium aluminate hydrate particles to cubic phase calcium aluminate particles; and
v) drying the slurry at a temperature suited to prevent thermal conversion of the hexagonal phase calcium aluminate hydrate particles to cubic phase calcium aluminate particles.
Another object of one embodiment of the present invention is to provide a method for synthesizing cubic phase calcium aluminate hydrate particles, comprising the steps of:
i) providing a source of calcium aluminate particles;
ii) hydrating the calcium aluminate sufficiently to effect hydration of at least 75% by mass of the particles in a slurry;
iii) agitating the slurry sufficiently for hydration to occur;
iv) maintaining the temperature of the slurry in a range sufficient to convert any hexagonal phase calcium aluminate particles formed to cubic phase calcium aluminate hydrate particles; and
v) drying the slurry.
By controlling the amount of unhydrated (anhydrous) material present, the inherent variability of the prior art mixtures is avoided. The complete hydration of 75% of the mass to form C3AH6 and CAH10 allows for full predictability in the final product.
A further object of one embodiment of the present invention is to provide a method for synthesizing cubic phase calcium aluminate hydrate particles, comprising the steps of:
i) providing a source of calcium aluminate particles;
ii) hydrating the calcium aluminate particles;
iii) curing the mixture in a humid atmosphere for a period sufficient to prevent self desiccation at a temperature sufficient to effect conversion of hexagonal phase particles to the cubic phase calcium aluminate hydrate particles; and
iv) drying the mixture.
A still further object of one embodiment of the present invention is to provide a method of forming a cementitious mixture having reduced shrinkage in use, comprising mixing the compounds:
i) cementitious material;
ii) an expansion effecting sulfate compound for effecting expansion in the mixture;
iii) calcium hydroxide; and
iv) presynthesized calcium aluminate hydrate particles formed by the method of claim 14.
An even further object of one embodiment of the present invention is to provide a method of forming a cementitious mixture having reduced shrinkage in use, comprising mixing the compounds:
i) cementitious material;
ii) an expansion effecting sulfate compound for effecting expansion in the mixture;
iii) calcium hydroxide; and
iv) presynthesized calcium aluminate hydrate particles formed by the method of claim 19.
As very briefly touched on earlier, plastic shrinkage occurs in the fresh (plastic) state while the concrete is setting and hydrating after its placement. It is aggravated by the incorporation of flyash and retarding admixtures. Both of these materials (flyash and retarders) are vital to offset deleterious cracking under hot weather conditions. One of the chief methods of overcoming this limitation is early and prolonged (three days) water curingxe2x80x94rather cumbersome in the field and, therefore, often avoided. In cementitious mortar, grouts and patching materials, aluminum flakes or fine particles of coke are used. These materials produce gas (H2 and air) in the fresh state which renders a buoyancy to the mix, thus reducing the settlement of the heavier stone in the mix, reducing plastic shrinkage.
These materials, however, are affected by the variations in field practice (water content of mix and varying ambient conditions). As a feature of one embodiment of the present invention, CAH10, when added to the concrete on mortar, will perform to minimize plastic shrinkage but without the limitations of:
potential hydrogen embrittlement (for aluminum powder); and
drastic reduction in expansion at lower temperatures (for coke).
Another object of one embodiment of the present invention is to provide a method of forming a cementitious mixture having reduced shrinkage in use, comprising mixing the compounds:
i) cementitious material;
ii) an expansion effecting sulfate compound for effecting expansion in the mixture;
iii) calcium hydroxide; and
iv) presynthesized calcium aluminate hydrate particles formed by the method of claim 27.
A further object of one embodiment of the present invention is to provide a method of forming a cementitious mixture having reduced shrinkage in use, comprising mixing the compounds:
i) cementitious material;
ii) an expansion effecting sulfate compound for effecting expansion in the mixture;
iii) calcium hydroxide; and
iv) presynthesized calcium aluminate hydrate particles formed by the method of claim 23.
Applicability of the technology set forth herein is widespread. As an example, cementitious mixtures where utility is clear include Portland cement, flyash, blast furnace slag, rice husk ash, type K, S and M cement, grout, stucco, shotcrete, patching and concrete among a host of others.
Having regard to the fact that distinct particles can be prepared (hexagonal, cubic or mixtures thereof), it is evident that full control of expansion is possible. Clearly this affords the user with the ability to change a specific mixture depending on specific requirements, environmental conditions etc., all of which were not previously available in the prior art methods and materials.
It is envisioned that the admixtures disclosed herein will have utility in borehold plugging, soil stabilization, mining, oil well cementing, flooring products, injection grouting, overlays, grouts, etc.
Having thus described the invention, reference will now be made to the accompanying drawings illustrating preferred embodiments.